1. Field of the Invention
This invention relates to polyester molding compositions and more particularly to compositions having improved physical appearances with respect to rippling waviness.
2. Prior Art
Unsaturated polyester resins have been employed commercially in various fiber reinforced fabrication systems including among others matched metal-die compression, transfer, and injection molding. These systems involve curing a formulated compound at high temperatures and pressures in hardened and polished molds. These methods provide the highest volume and highest part uniformity of any thermoset molding technique.
A technical improvement that has made a significant contribution to commercial thermosetting molding is the use of low profile additives to reduce shrinkage during the curing reaction and to thereby improve dimensional stability and surface smoothness. Low profile additives are, in general, thermoplastic polymers such as vinyl acetate polymers, acrylic polymers, polyurethane polymers, polystyrene, butadiene styrene copolymers, saturated polyesters and polycaprolactones.
A major advance in commercial thermosetting molding technology was the introduction several years ago of chemically thickened systems. Chemical thickening is always employed in sheet molding compounds ("SMC"), and is increasingly being used in bulk molding compounds ("BMC"). In such systems, and alkaline material such as magnesium oxide or magnesium hydroxide is added to, for example, an uncured polyester along with fillers, glass fiber, and other standard materials. The alkaline material interacts with residual acidity in the polyester to build viscosity. The thickened system is relatively tack free and easy to handle, and the high viscosity carries the glass fiber reinforcement to the extremities of the mold during crosslinking of the system. Thus, the use of thickened systems has made a major contribution to the commerical expansion of polyester molding.
While low profile unsaturated polyester fiber glass reinforced molding systems have gained wide acceptance in the transportation industry because of good surface appearance, dimensionsal stability, physical properties, production and assembly costs and weight savings versus metal, there is still a need for further improvement in reducing rippling and waviness.
In response to these increased demands the art has developed a variety of answers.
U.S. Pat. No. 4,525,498 teaches a low profile additive compostion containing a thermosetting unsaturated polyester, a thermoplastic polymer additive to control shrinkage and an ethylenically unsaturated monomer suitable for use in molding application the improvement being incorporating into said mixture and epoxy compound having at least one 1, 2 epoxy group per molecule said epoxy compound being essentially free of reactive unsaturation.
U.S. Pat. No. 4,374,215 teaches a polyester molding composition comprising:
(a) a polyester resin comprising the reaction product of an olefinically unsaturated dicarboxylic acid or anhydride and a polyol;
(b) an olefinically unsaturated monomer that is copolymerizable with said polyester resin;
(c) a thermoplastic polymer low profile additive to control shrinkage; and
(d) an effective amount of a second crosslinkable vinyl monomer having a reactivity ratio (r.sub.1) with styrene of greater than 1. The addition of component (d) in said U.S. Patent is stated to result in moldings with improved surface characteristics.
U.S. Pat. No. 4,673,706 teaches the addition of crosslinkable vinyl monomers and epoxy compounds to low shrinking polyester molding compositions containing unsaturated polyesters results in moldings with improved surface characteristics.
U.S. Pat. No. 4,755,557 teaches a molding composition comprising an unsaturated polyester resin, an ethylenically unsaturated monomer, copolymerizable with said polyester resin, a thermoplastic low profile additive wherein the improvement consists of adding lactone monomer at 2-20 parts per hundred of the combined weight of unsaturated polyester resin, ethylenically unsaturated monomer and thermoplastic low profile additive resulting in improved surface and shrinkage control.
In addition to the chemically thickened molding compositions mentioned above there is still a major and even growing interest in compositions that are not chemically thickened. These are widely used and are of particular interest in the high speed process of injection molding. Because of the inherent faster cycles and the ability to automate easier than compression molding this process is undergoing great growth for the production of automotive body panels from thermosetting unsaturated polyester fiber reinforced molding materials. Many times unthickened compounds are desired here because of the ability to control compound viscosity consistency better and longer shelf life of the material without requiring machine and molding parameter changes. Also in this type of material there is the need to obtain better surface smoothness and shrinkage control for increasing product demands.
A problem with these chemically unthickened molding compounds is that unless the unsaturated polyester resin structure and low profile additive structure are very carefully matched the organic materials tend to seperate in the compound upon storage. This can result in a phenomenon called "scumming" in which a haze or film is observed on the molded part and even on the mold itself. This is highly undesirable because it can cause part sticking in the mold, surface deterioration and possibly paint adhesion problems. The presence of this "scumming" is best analyzed by observing the shiny character or gloss of the molded part as whenever it is present this property is reduced.
This separation problem and scumming greatly limits unsaturated polyester resin and low profile additive structures that can be utilized and overcoming it would be a major improvement. Such an improvement could also apply broadly to other unsaturated polyester resin fabrication techniques such as pultrusion, resin transfer molding and compression molding.
Also there is a special application for these compositions and that is in the production of complex shaped headlamp reflectors parts for automobiles and trucks. The nature of this application makes it desirable that the compound have as low a coefficient of thermal expansion as possible to maintain constant light focus as the headlamp increases in temperature with use.